Such photoelectric sensing devices are able to detect any material object or article which penetrates, passes through, cuts or arrives into the transmission path between a light emitter and a light receiver or cuts the propagation direction of the light emitter. Thus such devices sense the decrease, increase or change of light radiation arriving at a photosensitive component from a corresponding light emitting component, and produce an output signal indicative of said decrease or change or of the presence of an object or article within the transmission path.
The light received by the photosensitive component can be either the light arriving directly from the light emitting component (when the two components are facing each other), or the light reflected by an object or article exposed to the light pulses from the first module.
These photoelectric sensing devices or sensor arrangements generally comprise at least a first module emitting light pulses with a given duration and at a given frequency or with a given period, a second module able to receive and convert the light transmitted by the first module and a third level judging and/or processing module.
In these devices or arrangements light pulses sent by said first module and received by said second module are converted by said latter into an analogue potential signal which is transmitted to the third module via a coupling or connecting capacity.
Said analogue signal is processed by said third module in synchronism with the light pulses, possibly by comparing at least its maximum value with a threshold, and an evaluation or control information or signal is provided by said third module depending on said processing results.
More precisely, the first module produces light pulses and transmits them to the second module. Said second module then exits or produces an analogue potential signal, corresponding to the light received of said second module, and transmits said signal to the third module through a coupling capacity. The analogue signal received through the coupling capacitor at the input of the third module is processed by this latter, possibly by comparing at least its maximum amplitude value with a threshold. Said third module provides an evaluation or control information (data) or signal depending on said processing results.
FIG. 1 of the drawings shows schematically the structure of such a known sensor arrangement.
As can be seen, the first module generally comprises a pulse generator and a light emitting block (for example a LED), possibly associated with a directing component. The second module may comprise a photosensitive block with a photoelectrical converter function, followed by an amplifier. Finally, the third module generally includes a processor unit, preceded or not by a level judging block. In order to synchronise the emitting and processing functions, the pulse generator of the first module preferably drives or sequences the functioning of the processor unit of the third module.
As can be seen from the chronograms a) and b) of FIG. 2, the light emitting component produces short periodical light pulses or flashes which are sent directly or indirectly (by reflection from an object when such an object is present) towards the photosensitive receiver. This operating mode allows to obtain a higher sensitivity of the sensor arrangement and to exclude the influence of a constant or low frequency illumination upon object detection.
The current driving pulses feeding the light emitting component [as shown on FIG. 2 a)], produce at the point X of FIG. 1 the signal represented on FIG. 2 b).
In the known sensor arrangements the signal readings occur within the time interval Δt and only the value of the signal during said interval are taken into account. Thus, the signal to be detected must exceed the threshold during said interval Δt to be acknowledged by the level judging block.
Nevertheless, it has been noticed that, especially in environments polluted with parasitical or interfering light radiations, the light pulses from the light emitting component can be masked, disformed, distorted or otherwise at least partially altered or suppressed, which leads to an unusable and unexploitable signal at the input of the third module (see FIG. 2 c)).
It is an aim of the present invention to overcome the aforementioned problem.